As Pam Summerhayes struggled for breath in the final hours of her young life, she summoned her big sister, Heather, for a last wish.

"Write ... our ... story," she commanded.

And this is where that story begins: My sister's voice is not her own.

"Tell ... what we ... lived through ... together."

A great energy washes through the room, as if a veil between two worlds has been lifted. My skin prickles with the sensation.

"Yes," I whisper. "I will."

She reaches out, fiercely grasping my hand. "PROMISE ME."

"I promise."

She releases her grip and falls back. A soft roar fills my ears; the hiss of the rain maybe, the whirr of my own blood racing, the cold hush of swift black water as my sister begins to drown and I am swept alone downstream.

It took Heather Summerhayes Cariou more than two decades to fulfill that promise, decades spent struggling to come to terms with life after loss. And as much as it's a testament to her sister, it's also an affirmation that life's journey goes on and that a first-time author can find success in her mid-50s.

"If there's anything I've learned, it's that life is valuable. You have to have purpose and treat it with respect," says Summerhayes Cariou, whose memoir, "Sixtyfiveroses: A Sister's Memoir," has just been published in the United States.

The title refers to how 4-year-old Pam pronounced "cystic fibrosis" when she was first diagnosed with the disease.

Her sister's diagnosis became a defining fact of Summerhayes Cariou's childhood, a fact that caused pain, disruption, jealousy and guilt, but also forged bonds of love, strength and wisdom, and a determination to seek the beauty in life.

"I think the feelings and experiences of the well siblings are as important as those of the parents," Summerhayes Cariou says of families with seriously ill children. "Siblings are our peers. We spend more time with them than we do our parents. They're the crucible in which we test our social skills."

She pauses, looking out the windows of her high-rise condo on the Hudson River in Guttenberg, the spectacular view of Manhattan obscured by clouds and rain on this day.

Inside, serenity reigns. Candles glow, family pictures and paintings line the walls, and cabinets are stuffed with curios and mementos she and her husband of 22 years, the actor Len Cariou, have collected. His Tony Award, for "Sweeney Todd," is on modest display on a bookshelf.

They moved from the city to New Jersey nearly a decade ago, to enjoy some sky and trees without paying Manhattan real estate prices. They decided upon their complex when they visited singer and actress Judy Kaye, who lives there with her husband, David Green.

"At first we were total snobs. I'm a bridge-and-tunnel girl now. I get 40 minutes of guilt-free reading on the bus and, when I come back here, my shoulders drop," Summerhayes Cariou he says, mimicking a sigh of relief.

Illness changes everything
Born in the mid-1950s two years apart to a young couple in a small town in Ontario, Heather and Pam were typical sisters in many ways. They put on shows, giggled and whispered after dark in the room they shared, and romped in the yard.

"Whether you're tied by hatred or love, when that sibling who hates you is gone, you're as much adrift as losing the sibling who loves you. You no longer have that ballast," she says.

They were each other's ballast.

Pam's illness changed everything. Always sickly and thin, she was diagnosed in 1956 with cystic fibrosis, a genetic disorder that afflicts 30,000 Americans in which the lungs fill with a thick mucus that leads to life-threatening infections. At the same time, the person is further weakened because the body obstructs the process by which it breaks down and absorbs food. Back then, it was a death sentence; few children with the disease lived to go to kindergarten.

Soon life was punctuated with hospitalizations and doctor visits. An oxygen tent to help Pam breathe became part of the girls' bedroom decor. Their parents, Donna and Douglas Summerhayes, founded the Cystic Fibrosis Foundation of Canada and spent hours upon hours working to raise awareness and funds to help fight the disease.

In many ways, their labors have borne fruit: Pam herself lived to her mid-20s, and a younger brother, Jeff, has lived with a milder form of CF into his mid-40s. The mean age of survival in 2006 was 37, according to the CF Foundation.

A keen awareness
Summerhayes Cariou is donating a portion of the books' profits to the Canadian foundation, and she often speaks at chapter events around Canada. She hopes to give more workshops and talks on the topic.

Heather became her sister's champion and confidante. "She was a funny, compassionate, very stoic person," she says of Pam. "I laughed with her like no one else. We had a secret language."

But she also resented all the attention given to her sister and felt keenly the guilt of the healthy survivor -- traits she later found were common when she researched studies for her book.

"The (healthy) kids are invested in not bothering the parents. They have to become preternaturally mature and assume responsibility," she says. For a strong-willed eldest child, that was a tough role to play, and it influenced her decision to become an actress, where the center of attention would shine solely on her.

"The choices you make have a lot to do with that experience," she says.

The knowledge of her sister's mortality also gave her an intensity that's a familiar trait to her family and friends. "When I was a child, I was aware of remembering everything. Time was of essence because Pam had such a short time," says Summerhayes Cariou, her blazing blue eyes punctuating her speech.

The book also captures the simple joys of family life in the ¤'50s and ¤'60s, the sights, smells and textures of pot roast dinners, Sunday services at the Baptist church, a starched new dress her mother wore on a night out.

And despite the unmet needs she grapples with, the Summerhayes family is portrayed as loving, loyal and rising to crises with dignity.

Both parents were understandably nervous when their daughter first told them she was working on a memoir.

"I told her I felt like I was the going to be the emperor who had no clothes, but that didn't last long," says her mother, Donna, a retired nurse. "(The book) captured our situation well, and we're secure enough in ourselves. Perhaps this serves as a lesson to some other families."

Pam started to decline in her 20s, the same time her sister was trying to launch her own life, attending theater school, trying to break into acting and marrying another young thespian.

"She was a stunningly beautiful gal, a very feisty and strong-willed young woman," says her friend of 30 years, Toronto-based actor Deborah Grover. "There was also a tremendous vulnerability about her. ... I think she felt very guilty. She had to realize, 'Yes, this is my journey.'"

'The guts of 10 burglars'
That journey sputtered after her sister's death, when she first met Cariou.

"I was working at the Stratford Festival and we met through a mutual friend," her husband remembers. "We weren't really looking for anyone. I was with someone at the time (actress Glenn Close) and, when I asked her to tell me something about herself, she said, 'Well, my sister died about six months ago, I got divorced four months ago, and I'm going on a white-water rafting trip in Oregon and I can't swim."

Cariou pauses with an actor's timing: "Oh, I said. Well, if you ever get back, give me a call."

She did. By that time, Cariou and Close had split up, and the two have been together since. "She's an extraordinary human being. She has the guts of 10 burglars," Cariou says.

Summerhayes Cariou went to New York in 1983 to be with Cariou, but she couldn't work in the United States. She talked about her promise to Pam, but the idea of writing terrified her, she says.

"Len said, 'You have everything you need to write this book, and what you don't have I'll support you with.' So, true to his word, he bought me an IBM Selectric and some paper. Of course, the joke is it took 20 years," she says with a laugh.

Her mentor, writing coach June Gould, met her 20 years ago at the annual workshop of the Women's International Writing Guild, held each June at Skidmore College outside Albany.

"She didn't have a book, just her sister's desire to have her do it. She showed me the first few paragraphs, and she was really nervous. She had no idea if she had any talent," recalls Gould, the author of "The Writer in All of Us." "But I could immediately see she had talent."

The support to go on
These days, Summerhayes Cariou is taking a master class with Gould in New York, polishing a novel.

Summerhayes Cariou says her writing is informed by her acting background, by exploring intent through actions and by using techniques of Gould's, focusing on objects that bring the past to life "like Technicolor." Through many drafts, it transformed from a piece of straight reporting to a novel-like treatment.

The guild, founded 30 years ago, is dedicated to nourishing women writers, and Summerhayes says she found a spiritual home with the group. "They just held me up and would not let me quit. They made me believe in myself," she says.

The other support was Cariou: "He never rushed me, never asked me to justify my creative existence," she says. "He never said, 'Have you given up? When are you getting back to it?'"

Her writing was interrupted by periods of mourning for her sister, as well as by the demands of her other job: as "roadie" to her husband when acting gigs take him away from home. Last year, the couple spent only 12 weeks in their apartment, and not continuously, as Cariou filled engagements in Canada and the United States, most recently a month at the Manitoba Theater Center in Winnipeg.

"I travel like a refugee. I take tablecloths, candlesticks, sheets and towels, photographs and paintings," she says, laughing. Because they spend so much time in Los Angeles for film and television work, they rent a storage unit there containing duplicates of things from home.

And that's the point, she says, to create a home out of a corporate apartment.

"We're old enough not to want to live like college students," she says.

While Cariou grumbles that she overdoes it, he admits it's a nice environment to come home to after a day of shooting. A ritual for them is a roast chicken dinner wherever they are that announces they are settled, Summerhayes Cariou says.

"She's a great cook," Cariou confirms, and the couple like to entertain in their aerie overlooking the river.

They also get to Canada as often as possible to visit with nieces and nephews and Len's daughter, son-in-law and grandchildren, who call Summerhayes Cariou "Grandma."

These days she works out of a home office crammed with stuff of significance to her -- including, of course, stacks of crisp new books to sign. On each cover is a photo-booth snapshot of Pam and Heather in their early 20s, their eyes bright, their smiles wide.

"You know, a friend of mine was talking to me about the book, and he said maybe I was thinking that when I finished it, I'd have to say goodbye to Pam," she says softly. "But the relationship is stronger. I feel she's with me."